Cellulosic fibrous structures, such as paper, are well known in the art. Such fibrous structures are in common use today for paper towels, toilet tissue, facial tissue, etc. To meet the needs of the consumer, these fibrous structures must balance several competing interests. For example, the fibrous structure must have sufficient tensile strength to prevent the fibrous structure from tearing or shredding during ordinary use or when relatively small tensile forces are applied. The cellulosic fibrous structure must also be absorbent, so that liquids may be quickly absorbed and fully retained by the cellulosic fibrous structure. The cellulosic fibrous structure should also exhibit sufficient softness, so that it is tactilely pleasant and not harsh during use. Against this backdrop of competing interests, the fibrous structure must be economical, so that it can be manufactured and sold for a profit, and yet still be affordable to the consumer.
Tensile strength, one of the aforementioned properties, is the ability of the fibrous structure to retain its physical integrity during use. As discussed by D. H. Page, "A Theory for the Tensile Strength of Paper", TAPPI, Vol 52(4), p. 674-82 (1969), tensile strength is controlled by two primary factors: fiber zero span tensile strength and fiber-fiber bonding (affected by, e.g., fiber sheer strength, relative bonded area, fiber length, fiber cross sectional area, and the average perimeter of the fiber cross section). With tissue and towel products and the like, the fiber zero span tensile strengths are generally on the order of at least 10 times greater than the overall tensile strength of the sheet. This in turn indicates that factors which influence fiber to fiber (i.e., interfiber) bonding control the tensile strength of the web and that the zero span strength of the fiber (i.e., intrafiber strength) can be reduced without adversely affecting overall product strength.
Softness is the ability of a fibrous structure to impart a particularly desirable tactile sensation to the user's skin. In general, softness is inversely proportional to the ability of the fibrous structure to resist deformation in a direction normal to the plane of the structure. Softness is influenced by bulk, surface texture (crepe frequency, size of various regions and smoothness), the stick-slip surface coefficient of friction, and bending stiffness or drape (also referred as hand feel). One or more of these properties can be affected by fiber flexibility, fiber morphology, bond density, unsupported fiber length, and the like.
Not surprisingly, significant effort has been expended to enhance the tensile strength (wet and/or dry) of fibrous substrates; the patent literature is reflective of this effort. Examples of prior art means for increasing tensile strength are addition of chemical wet and dry strength agents, binder fibers such as bi-component fibers, latex binders, and the like. Similarly, significant effort has been expended to provide substrates having improved hand feel, or softness. Examples include addition of chemical softeners, surface modifying agent, debonding agents, and the like. Other examples include mechanical treatment such as creping, Clupak.RTM., Micrex.RTM., wet microcontraction, and the like.
It is generally accepted that the strength of a fibrous substrate (typically measured in terms of wet and/or dry tensile strength) and that substrate's softness are dependently related, at least to some degree. That is, efforts directed at enhancing substrate softness typically will result in a reduction in substrate strength. Indeed, many prior attempts to improve substrate softness have focused on modifying (reducing) fiber-to-fiber bonds via chemical and/or mechanical treatments such as creping. While softness benefits are achieved, a reduction in interfiber bonding gives rise to a reduction in substrate tensile strength and an increase in product lintiness. Thus, there is a continuing need for a means to decouple the relationship between substrate softness and strength. In particular, there is a need for fibrous products having improved hand feel without sacrificing web strength.
Accordingly, it is an object of the present invention to provide a fibrous web, comprising cellulose-based fibers, which exhibits improved softness without negatively impacting strength to a significant degree. This is achieved by preparing the webs using modified cellulosic fibers that have reduced zero span tensile strength (i.e., reduced intrafiber strength), as opposed to reducing the level of interfiber bonding (i.e., interfiber strength) of the web. More specifically, Applicants have discovered that a measurable reduction in the dry zero span tensile of fibers typically provides a fibrous structure that exhibits improved flexibility (as measured in terms of a reduction in "bending modulus per unit dry tensile"). While a reduction in dry zero span tensile strength does not always provide improvements in structure flexibility, such a reduction is believed necessary to achieve more flexible structures in accordance with the present invention.
It is a further object of the present invention to provide the above-described modified cellulosic fibers, as well as a process for obtaining the modified cellulosic fibers.